Balance, or a state of equilibrium, may be described as the ability to maintain the body's position over its base of support. In particular, the optimal posture for controlling balance typically requires maintaining the body's center of gravity (COG) within the base of support, such as the support frames defined by the soles. Balance may be divided into static balance and dynamic balance, depending on whether the base is stationary or moving.
Disequilibrium and movement and balance disorders can be debilitating and increase the potential for falls. A movement disorder is a condition that prevents normal movement. Some movement disorders are characterized by lack of movement, and while others are characterized by excessive movement. A balance control disorder is typically the result of sensory and/or motor disorders which impair equilibrium control by a subject. Balance control disorders may be bilateral, i.e., affect a subject on both left and right sides, or may only be manifested on one side. Movement and balance disorders may be caused by disorders in the vestibular, somatosensory, or central or peripheral nervous systems.
The vestibular system carries sensory information related to body equilibrium, specifically roll, pitch, and yaw motion oriented relative to the direction of gravity. Information is generated by the semicircular canals and maculae in the inner ear, relayed by the vestibular nerve to the brainstem vestibular nuclei, and processed by the vestibular nuclei and mid brain with corresponding muscular contraction and relaxation known as motor output.
Aspects of the somatosensory system include: 1) perception of pressure, vibration, and texture, i.e., discriminative touch, 2) perception of pain and temperature, and 3) proprioceptive sensation. Proprioception, which is often referred to more generally as the somatosensory system, involves awareness of movement derived from muscular, tendon, and joint articular surfaces provided by the peripheral nervous system and processed in the parietal lobe of the brain. These interoception senses provide internal feedback on the status of the body, indicating whether the body is moving with required effort and indicating where various parts of the body are located in relation to each other. Thus, proprioception involves the essential stimuli provided to, or received by, skin, joints, and/or muscles to maintain equilibrium or balance control.
Damage to any part of the central or peripheral nervous systems may interfere with balance control. Central nervous system processing includes the brain primary motor cortex responsible for generating the neural network impulses controlling execution of movement, the posterior parietal cortex responsible for transforming visual information into motor commands, the premotor cortex responsible for sensory guidance of movement and control of proximal and trunk muscles of the body, and the supplementary motor area responsible for planning and coordination of complex movements such as coordinated activity using two hands.
In particular, vision plays a significant role in balance. Indeed, up to twenty percent of the nerve fibers from the eyes interact with the vestibular system. A variety of visual dysfunctions can cause disequilibrium. These dysfunctions may be caused directly by problems in the eyes, or may be caused indirectly by disorders related to stroke, head injury, vestibular dysfunction, deconditioning, decompensation, or the like.
Meanwhile, the peripheral nervous system generally relates to the conduction of sensory information, or messages, from the peripheral nerves to the brain and spinal cord. For example, such sensory information may indicate that there is a pressure on the sole of a foot or that a toe is flexed. Sensory information may also indicate that the feet are cold or that a finger is burned. Peripheral neuropathy relates to defects in the peripheral nervous system. In general, damage to the peripheral nervous system interferes with the communication of messages to the brain and spinal cord.
Accordingly, the body relies on the interaction of several systems to control movement, balance, and posture. For example, the vestibular system in the ears orient upright stance, especially when the eyes are closed. The cutaneous, proprioceptive sensory system feels pressure under the feet. In addition, the joint and muscle spindles are sensitive to joint position and movement. Moreover, cognition or brain processing estimates the motor response magnitude. In sum, balance disorders are predominantly multi-causal with imbalance occurring due to deficits in more than one sensory, motor, neuro or cortical pathway.
The cause and extent of any deficits in a subject's movement and balance control may be determined by assessing the subject's ability to control movement and balance while performing a number of standard functional motor tasks, such as standing still, moving from a sitting position to a standing position, walking, walking on steps and uneven surfaces, or the like. This assessment may be achieved by manipulating sensory input and monitoring motor response. Quantified sensory assessment, for example, may examine touch-pressure, two-point discrimination, inner ear response to warm and cold, or visual acuity by reading the print on an eye chart. Diagnosis may also be determined qualitatively according to the observations by an examining physician or a physical therapist.
After a balance deficit has been diagnosed and quantified, a physician may prescribe remedial measures to try and bring the subject's balance control near or within normal limits. In certain instances, the physician may prescribe medication that reduces the action of peripheral senses on the brain or enhance neural network function. Alternatively, the physician may prescribe a course of physical therapy, which will typically last at least several months, with the object of training the subject's brain to deal with a reduced sense of balance when trying to maintain the body upright and prevent a fall. Normally, neither of these techniques will have an immediate effect on the subject's balance deficit. Moreover, medication can have side effects, and can also reduce the capability of the brain to process balance information from the peripheral senses. A traditional course of physical therapy requires a long training period which may extend over more than two months. These difficulties and limitations associated with conventional remedial measures for dealing with balance deficits are most problematic when the subject is older and likely to have a falling tendency.